Salmonella typhimurium possesses an adaptive response to acid that increases survival during exposure to extremely low pH values. The acid tolerance response (ATR) includes both log-phase and stationary-phase systems. The log-phase ATR appears to require two components for maximum acid tolerance, namely an inducible pH homeostasis system, and a series of acid-shock proteins. We have discovered one of what appears to be a series of inducible exigency pH homeostasis systems that contribute to acid tolerance in extreme acid environments. The low pH-inducible lysine decarboxylase was shown to contribute significantly to pH homeostasis in environments as low as pH 3.0. Under the conditions tested, both lysine decarboxylase and sigma s-dependent acid-shock proteins were required for acid tolerance but only lysine decarboxylase contributed to pH homeostasis. The cadBA operon encoding lysine decarboxylase and a lysine/cadaverine antiporter were cloned from S. typhimurium and were found to be 79% homologous to the cadBA operon from Escherichia coli. The results suggest that S. typhimurium has a variety of means of fulfilling the pH homeostasis requirement of the ATR in the form of inducible amino acid decarboxylases.

In Salmonella enterica serovar Typhimurium, the membrane-localized CadC is a transcriptional activator of the cadBA operon, which contributes to the acid tolerance response. Unlike in Escherichia coli, in which transcription of cadC is constitutive, in S. enterica serovar Typhimurium cadC expression is induced by low pH and lysine. Inactivation of cadC suppresses the acid-sensitive phenotype of a cadA mutation, suggesting the existence of other CadC-dependent genes in addition to the cadBA operon. Using a proteomic approach, we identified 8 of the putative CadC-induced proteins and 15 of the putative CadC-repressed proteins. The former include porin proteins OmpC and OmpF. The latter include proteins involved in glycolysis, energy production, and stress tolerance. To better understand the altered levels of OmpC and OmpF, we compared expression of ompR in S. enterica serovar Typhimurium wild-type and cadC mutant strains and determined that CadC exerted a negative influence on ompR transcription. Taken together, our findings strongly suggest that CadC may be a global regulator involved in the OmpR regulatory system during acid adaptation.

Gram-negative bacteria, including Salmonella enterica serovar Typhimurium, exploit type III secretion systems (T3SSs) through which virulence proteins are delivered into the host cytosol to reinforce invasive and replicative niches in their host. Although many secreted effector proteins and membrane-bound structural proteins in the T3SS have been characterized, the functions of many cytoplasmic proteins still remain unknown. In this study, we found that IacP, encoded by Salmonella pathogenicity island 1, was important for nonphagocytic cell invasion and bacterial virulence. When the iacP gene was deleted from several Salmonella serovar Typhimurium strains, the invasion into INT-407 epithelial cells was significantly decreased compared to that of their parental strains, and retarded rearrangements of actin fibers were observed for the iacP mutant-infected cells. Although IacP had no effect on the secretion of type III translocon proteins, the levels of secretion of the effector proteins SopB, SopA, and SopD into the culture medium were decreased in the iacP mutant. In a mouse infection model, mice infected with the iacP mutant exhibited alleviated pathological signs in the intestine and survived longer than did wild-type-infected mice. Taken together, IacP plays a key role in Salmonella virulence by regulating the translocation of T3SS effector proteins.

No abstract

The type III secretion systems (T3SSs) are exploited by many Gram-negative pathogenic bacteria to deliver a set of effector proteins into the host cytosol during cell entry. The T3SS of Salmonella enterica serovar Typhimurium is composed of more than 20 proteins that constitute the membrane-associated base, the needle and the tip complex at the distal end of the T3SS needle. Membrane docking and piercing between the T3SS and host cells is followed by the secretion of effector proteins. Therefore, a secretion hierarchy among the substrates of the T3SS is required. The secretion of the pore-forming translocase proteins SipB, SipC and SipD is controlled by the T3SS regulator protein, InvE. During an attempt to identify the regions of InvE that are involved in T3SS regulation, it was observed that the secretion of SipB, SipC and SipD was inhibited when the C-terminal 52 amino acids were removed from InvE. In addition, InvE derivatives lacking the Nterminal 30 and 100 residues were unable to secrete translocases into the culture medium. Interestingly, in the absence of the N-terminal 180 residues of InvE, SipD is unstable, resulting in the hypersecretion of SipB. We also found that both the type III secretion signals of SipB and SptP were functionally interchangeable with the first 30 amino acids of InvE, which could allow the secretion of a reporter protein. These results indicate that InvE may have two functional domains responsible for regulating the secretion of translocases: an N-terminal secretion signal and a Cterminal regulatory domain.